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Explain further that the role of marijuana in the treatment of chronic pain remains controversial and is not currently endorsed by federal authorities.

Smoking marijuana modestly reduced pain and other symptoms of chronic neuropathic pain, results of a small randomized, placebo-controlled trial showed.

The most potent dose used reduced average daily pain scores by 0.7 points on an 11-point scale (5.4 versus 6.1 with placebo, 95% confidence interval for difference 0.02 to 1.4), according to Mark A. Ware, MBBS, of McGill University in Montreal, and colleagues.

Those who smoked weed with 9.4% of the active ingredient tetrahydrocannabinol (THC) also reported sleeping better, the researchers reported online in CMAJ.

These results are important in light of the fact that patients who hear about pain relief from ongoing publicity about medical marijuana have had only a "trickle" of evidence to prove it, explained Henry J. McQuay, DM, of Oxford University, in an accompanying editorial.

"If medical cannabis is not available where a patient lives, then obtaining it will take the patient outside of the law, often for the first time in his or her life," he wrote. "Good evidence would at least buttress that decision."

These quality results along with three other trials of smoked cannabis for neuropathic pain do support an analgesic effect that, "though not great, may be of use to some patients," McQuay concluded.

This study does offer hope since few drugs have proven effective in these patients, commented Steven P. Cohen, MD, who as director of pain research at Walter Reed Army Medical Center in Washington, D.C., sees chronic pain in most of his patients with major war injuries.

However, the magnitude of the pain relief from smoking marijuana was less than expected compared with those few effective drugs, Cohen noted in an e-mail to MedPage Today and ABC News.

"When considered in the context of the higher incidence of minor and serious side effects with medical marijuana, cannabinoids should remain a third or fourth line drug for neuropathic pain," he wrote.

A bigger concern remains the "delivery system," which is substantially worse than tobacco cigarettes due to prolonged exposure to marijuana smoke from holding it in the lungs, commented Timothy A. Collins, MD, of Duke University's Pain and Palliative Care Clinic.

"As a physician, I don't think I would ever encourage a patient to smoke anything," he said in an e-mail.

Some states, such as California and New Jersey, have made marijuana "legal" for medical use, but this conflicts with federal law, and the U.S. Drug Enforcement Agency doesn't recognize its legality in those states.

"In states where there is no 'medical marijuana law' patients buy marijuana from criminals (drug dealers)," Collins added. "For pain patients this becomes a problem, as they are already on desirable prescription pain medications and subject to pressure to trade prescription narcotics for marijuana."

For the new study, Ware's group randomized 23 adults with chronic post-traumatic or postsurgical neuropathic pain to receive cannabis at a potency of 0%, 2.5%, 6% or 9.4% THC over four 14-day periods in a crossover trial.

The patients inhaled a single 25-mg dose through a pipe three times a day for the first five days in each cycle, then had a nine-day washout period before going on to the next randomized potency.

The intermediate doses yielded pain relief between the 0% THC placebo and the full-strength 9.4% dose. However, none were significantly effective compared with placebo.

Among the secondary effects, patients reported falling asleep more easily (P=0.001) and more quickly (P<0.001) with a greater feeling of drowsiness (P=0.003) and less wakefulness (P=0.01) while on 9.4% THC compared with placebo.

Mood and quality of life overall, though were unaffected.

Marijuana smoking appeared well tolerated, although the researchers noted that this may have been affected by the fact that most patients reported prior experience -- although not recent or extensive -- with the drug as an early ethics requirement to enter the study.

Patients rarely got high on the single hit they took through a pipe three times a day as part of the study, Ware's group noted.

None of the analgesic doses got plasma levels even halfway to the typical level seen among recreational users, they explained.

The most common drug-related adverse events associated with the most potent dose included headache, dry eyes, burning sensation in areas of neuropathic pain, dizziness, numbness, and cough.

Collins cautioned that marijuana bought on the street, not through medical-use prescription, varies widely in potency, making the study results of unclear generalizability to the average user.

This article was developed in collaboration with ABC News.

The trial was supported by a grant from the Canadian Institutes of Health Research and by the Louise and Alan Edwards Foundation for pharmacokinetic assays.

The researchers declared that they had no conflicts of interest.

McQuay reported having been a member of advisory boards for Reckitt Benckiser, Pfizer, and Archimedes; a consultant to Esteve, Sanofi and Archimedes; and an expert witness for Ratiopharm and Sandoz. He reported having received payment for lectures from Archimedes and Grunenthal and royalties for a textbook on the subject of pain.

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